The invention relates generally to seal assemblies for use in sealing components of oil and gas wells. In particular, the invention relates to a seal assembly for a casing hanger or a tubing hanger of a wellhead, the seal assembly having a system for axially restraining the seal assembly within the wellhead and a method for installing the seal assembly in the wellhead.
Casing is strong steel pipe that is used in an oil and gas well to ensure a pressure-tight connection from the surface to the oil and/or gas reservoir. Casing serves many purposes in the well. It can protect the wellbore from caving in or being washed out. It can also confine production to the wellbore, so that water does not intrude into the wellbore or drilling mud intrude into the surrounding formations. It can also provide an anchor for the components of the well. Production tubing, such as coiled tubing, typically is used to transport oil to the surface.
Several sections of casing joined together end-to-end are known as a “casing string.” Because casing serves several different purposes, it is typical to install more than one casing string in a well. Casing strings typically are run in a concentric arrangement, similar to an upside-down wedding cake, which extends downward into the ground from a wellhead located at the surface. For example, the casing string with the greatest diameter is the shortest, followed by smaller diameter casing string which goes deeper than the first casing string, followed by an even smaller diameter casing string that is run even deeper than the second casing string.
A casing hanger is a device that is used to support a casing string from a wellhead or other structure within the well. In addition, the casing hanger serves to ensure that the casing is properly located in the well. When the casing string has been run into the wellbore, the casing string is hung, or suspended, in the well by the casing hanger. Typically, the casing hanger rests on a landing shoulder inside the wellhead. Multiple casing hangers may be supported within a single wellhead.
Seals are used to seal the annulus between the casing hanger and the wellhead to isolate fluids in different casing strings from each other and to prevent production fluid from being diverted from its desired path. However, forces in the well may be produced that urge the seals upward, potentially causing the seals to leak. This also may lead to the seal unseating and moving relative to the casing hanger or tubing hanger. For example, fluid pressure in the annulus may urge the seal upward or downward. Similarly, thermal conditions in the annulus may exert forces that urge the seal upward.
One method that has been attempted to restrain axial movement of the casing hanger seal and casing hanger is by installing a latch ring on the casing hanger. The latch ring extends between the casing hanger and wellhead. The latch ring is activated by the installation of the seal. Unfortunately, a latch ring that is installed on a casing hanger will restrict the flow of fluid in the annulus surrounding the casing hanger during cementing operations. The casing string is cemented in position within the wellbore of the well. Cement is poured down the interior of the casing string and flows out the bottom of the casing string into the annulus between the casing string and its nearest concentric drill string. Fluid in the annulus is displaced upward through the annulus. The casing hanger latch ring interferes with the flow of this fluid.
Another method that has been used to restrain axial movement of the casing hanger and casing hanger seal is the use of wickers. Wickers are located on the outer surface of the casing hanger and inner surface of the casing hanger seal. Similar wickers are located on the outer surface of the casing hanger seal and the inner surface of the wellhead. The seal material is softer than the wellhead and casing hanger materials. When the seal is activated, the softer material of the seal is driven into the wickers of the casing hanger and wellhead. The engagement between the various wickers limits axial movement of the casing hanger and casing hanger seal relative to the wellhead. However, there are physical limits to the pressures and thermal conditions that this wicker configuration can be used to provide axial restraint. These physical limits may be exceeded with the higher pressures and temperatures associated with deepwater subsea wells.
Sill another method that may be used is to provide a separate lockdown sub dedicated to providing axial restraint for the casing hanger seal and casing hanger. However, the installation of a lockdown sub requires an additional trip of the drill string into and out of the well, a considerable expense. In addition, the lockdown sub occupies space, which is limited in a wellhead. Finally, the lockdown sub does not provide the axial restraint immediately upon installation of the casing hanger and casing hanger seal.
Therefore, an improved technique for preventing axial movement of a casing hanger and casing hanger seal within a wellhead is needed. In particular, a technique is needed to prevent axial movement of a casing hanger or tubing hanger seal within a wellhead immediately upon installation of the casing hanger and casing hanger seal, without interfering with the flow of fluids in the annulus surrounding the casing hanger during cementing operations, without occupying additional space in the wellhead, or requiring additional trips into the well.